The present invention relates to polymeric compositions exhibiting a combination of high gloss and high environmental stress crack resistance, especially high impact polystyrene compositions. More particularly, the invention relates to high impact polystyrene compositions containing rubber modified vinyl aromatic polymers and certain particular polyolefins, to molded and thermoformed articles made from such compositions, and methods for their production. Furthermore, the invention relates to composites with a gloss cap layer made from the polymeric compositions, and methods for their manufacture.
It is well known to make high gloss compositions from rubber modified vinyl aromatic polymers, such as for example high impact polystyrene. A drawback of such compositions, however, is that they are very susceptible to stress cracking when exposed to environmental oils, fats, detergents, or cleaners. Thus the known compositions are deficient in the property of environmental stress crack resistance, even though they have good gloss properties.
The environmental stress crack resistance of such compositions has been addressed in the prior art For example, in U.S. Pat. No. 5,543,461, the environmental stress crack resistance of impact modified styrenic polymers was increased by increasing the particle size of the impact modifier, and in U.S. Pat. No. 4,144,204, high impact polystyrene compositions with increased environmental stress crack resistance were produced wherein the rubber particles had a diameter of at least 4 microns. In U.S. Pat. No. 4,939,207, blends of a vinyl aromatic polymer, a polyolefin, and a vinyl aromatic monomer conjugated diene star block copolymer were provided which had good chemical resistance. However, in all of the above prior art examples, the high gloss properties of the resulting compositions are lost.
As a result, the known compositions do not provide high impact polystyrene compositions exhibiting a combination of both high gloss and desirable environmental stress crack resistance. Such compositions would be particularly advantageous for thermoforming or sheet extrusion as a gloss cap layer on co-extruded sheet for food packaging or refrigerator liner applications, or for example as molded articles such as telephone handsets subject to attack by the natural oils in the skin.
According to the invention, there is now provided a polymeric composition exhibiting a combination of high gloss and high environmental stress crack resistance, comprising
(a) a high gloss rubber modified vinyl aromatic polymer; and
(b) a polyolefin that has a stress exponent less than 1.70,
wherein the gloss at 60 degrees of the composition is greater than about 85%, and the environmental stress crack resistance measured in minutes until breakage at 1000 psi is more than about 60.
Surprisingly, it has been found that when a particular polyolefin (b) having a stress component of less than 1.7 is used, the resulting composition has an advantageous combination of high gloss and a desirable level of stress crack resistance.
Advantageously, the polymer composition further comprises a compatibilizing polymer (c).
In another embodiment, the polymer composition further comprises a low gloss polymer (d).
The invention also provides molded thermoplastic articles made from the above polymeric composition.
In addition, there is provided an extruded thermoplastic sheet from the above composition, as well as thermoformed articles made from the sheet.
Furthermore, there is provided a multilayer thermoplastic composite comprising a substrate layer and a gloss cap layer made from the above polymeric composition.
In addition, there are provided methods of making the articles, sheets, and composites mentioned above.